The ornamental design for a hairband, as shown and described.
FIG. 1 is a front left perspective view showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left view thereof;
FIG. 5 is a right view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is a rear right perspective view thereof; and,
FIG. 9 is an enlarged fragmentary view.
The stippling is used to depict contour. The dash-dot-dash circle in FIG. 7 is not part of the claimed design and shows the fragmentary view in FIG. 9.